If you didn't add credits to the image or at the top of your bio, that's what you need to do.
The only way you can use art here, is if it's 100% properly credited.
And if you add it to your bio, you'll be required to keep them in your bio for as long as you have the avatar in use. If you remove the credits, the image will be removed as well for having no credits.
If you did add your credits, they probably were just too small/illegible to read. You'd need to enlarge them/make them more noticeable/easier to read if this is the case.
However, depending on what the image was of, it's possible that the issue was something else entirely
Fall Month: 3 
Night 
Hurricane Moon: 
